Trouble weight painting character
If I select the shoulder bone (the one that affects the mesh and that is on the first armature layer, not the one on the third armature layer), you can see that it influences the neck area (and also the head) when it should not.
Other bones affect the head as well. You need to correct this, for example with the weight paint brush in Subtract mode.
Also maybe simplify your mesh, it will make things easier, unless you have some good reason not to use a Subdivision Surface modifier. And why not simplify the armature as well?